This series represents a mini course in dental alloys for the beginner, and persons seriously interested in gaining a basic working knowledge of dental alloys are advised to take the time to start at the beginning.
If all five pages are read in order, the reader will gain a good understanding of just what dental alloys really are, their internal crystalline structures, how they differ from each other and how different alloys are utilized in various applications.
Metal Castings are made by fabricating a hollow mold, pouring a molten metal into it, allowing the metal to solidify and separating the now solid metal casting from the mold. Ultimately, all metallic objects originate from castings. In dentistry, metal castings are used to restore teeth, replace teeth, and as frameworks for removable partial dentures. Today, metal castings are also used as metal frameworks to support porcelain crowns or fixed partial dentures in order to produce strong, and yet very esthetic restorations.
The lost wax technique was probably invented in ancient China or Egypt. The technique consists of carving a wax replica of an item that is to be duplicated in gold. The wax is invested (imbedded) in plaster or clay and burned out leaving an image (hole) where the wax used to be. Then the image is filled with molten gold through a small hole in the investment. This technique works quite nicely for fairly large castings, but gravity alone is not sufficient to draw gold into the very fine detail necessary to fabricate a tiny filling for a tooth.
Prior to 1855, dentistry consisted mostly of extracting decayed and abscessed teeth and replacing them with some sort of removable denture. Silver amalgam, made from shaved silver coins mixed with mercury, was invented in France in 1819, but was an unreliable filling material due to the haphazard way it was formulated. While itinerant entrepreneurs traveled the countryside plugging amalgam into decayed teeth, most reputable dentists refused to use it. Gold leaf was first used to fill teeth in about 1483 by Giovanni d'Arcoli, but the technique was extremely tedious and expensive and only the most wealthy and determined patients could afford and withstand having their decayed teeth repaired this way.
The cohesive gold foil technique was perfected and codified in 1855. It was much less tedious and less expensive than using gold leaf and made restoration of decayed teeth a real option for a wider swath of consumers. The gold foil technique consists of a rather laborious and still fairly expensive process involving hammering tiny pieces of pure (cohesive) gold foil into an equally laboriously prepared cavity preparation. Only reasonably affluent people could afford this sort of dentistry, but it was reliable, and gold foil became the industry standard for repairing damaged teeth. In 1895, G.V. Black standardized a reliable and safe formula for dental amalgam . This made it possible for the average (non wealthy) person to save a decayed tooth rather than having to extract it. Unfortunately, not all dentists were on board with the mercury, so many remained wedded to the gold foil technique.
In 1907, William H. Taggart invented a centrifugal casting machine for use with the lost wax technique. With centrifugal force replacing gravity as a method of filling the casting image inside an investment, it became possible to cast small, highly detailed objects. He worked up the procedures for the technique and patented it, however he eventually lost the patent when it was discovered that a Dr. Philbrook of Denison, Iowa had published a paper on the subject twenty five years before. Taggart's procedure involved carving a wax pattern directly inside of a patient's open mouth. Today, a dentist takes an impression and sends it to a dental laboratory. The lost wax technique is explained below:
A dentist laboriously drills out the decay from a tooth and refines the shape of the preparation, being careful that there are no undercuts which might interfere with an unrestricted path of withdrawal.
The dentist then takes an impression of the prepared tooth. This impression is then sent to the lab for fabrication of the restoration. The images presented here show how a gold crown is fabricated in the laboratory.
After pouring the impression with a fine plaster called dental stone, the die (the plaster model of the prepared tooth) is then covered with wax and then carved into the appropriate tooth shape. (Thanx to Bothell dental lab for the use of their images.)
Then, a sprue (simply a small wax rod) is attached to the wax replica. In the image below, the sprue is the green extension from the crown down toward the casting ring cap at the bottom. The bulb in the sprue serves as a reservoir for the gold to help equalize the pressure of the liquid gold so it flows evenly into each wax pattern
The casting ring cap is then fitted over the casting ring. The casting ring serves as a container to hold the plaster (actually in improved investing plaster) which is then flowed around the wax patterns.
Once the investment has set, the casting ring cap is removed leaving the sprues sticking up out of the now hard investment. The cylinder, with its invested wax, is placed in a very hot oven. When the wax burns away, the plaster in the ring then contains a space in the shape of the original wax filling (actually a hollow three dimensional image of the filling with attached sprue).
The image is then filled with molten gold using a centrifugal casting machine.
By immersing the still hot plaster with its gold innards in water, the plaster would shatter away leaving behind the casting which includes the gold filling and the attached sprue.
After removing the sprue, the gold casting was polished up and cemented into the original cavity preparation in the tooth.
This technique works equally well for fillings in teeth as well as full gold crowns. When a casting does not replace the cusp of a tooth, it is called an inlay:
When a casting replaces one or more cusps on a tooth it is called an onlay:
Taggart's centrifugal casting machine made it possible to apply enough "gravity" to force the molten gold into the tiny invested image of a filling. The picture on the left shows a modern version of Taggart's invention. The white piece with the hole in it is a small crucible used to melt the gold alloy with a gas and forced air torch. The burned out image (originally invested in a metal cylinder, called a casting ring), is placed behind the hole in the crucible. The orange stand contains a spring which has been wound several times in preparation for the casting operation. Once the image is in place and the alloy has been melted, the technician allows the locking pin that sticks up on the left side of the base to drop. This releases the armature, and when the technician lets the armature loose, the armature, along with the crucible and its attached casting ring spins at considerable speed. The crucible apparatus swings out so that it is facing the counterweights on the opposite side of the armature. Centrifugal force forces the melted alloy through the hole in the crucible and molten metal proceeds to fill the image in the casting ring behind it.
Unfortunately, Taggart's technique did not produce the accuracy that many dentists demanded for these small restorations, so most dentists still resisted the introduction of cast metal restorations in favor of gold foil or the newly improved silver amalgam, both of which always produced the tightest restorations possible. Still, even in the 1910's, wealthy people wanted high class dentistry and were willing to pay more for the privilege of not having to sit around suffering while the dentist hammered gold into the cavity preparation. Thus, cast gold restorations began to compete successfully with gold foil almost immediately, in spite of the fact that the castings did not fit the preparation perfectly. Since gold was the metal used to make the crowns worn by kings, it suited the mentality of the day to think of being able to afford the services of an expensive dentist as something that brought a royal distinction to the patient. Thus the term "gold crown" was something like an advertising slogan. The term "crown" was used to denote any gold restoration applied to a single tooth, including gold foil restorations, inlays and onlays. Today, the term "crown" is reserved for any full coverage restoration, whether gold or porcelain, and the terms "inlay", "onlay" and "filling" are used to denote restorations that cover only a part of a tooth's clinical crown.
In 1929, Coleman and Weinstein invented cristobalite investment to replace the plaster of Paris, eliminating most of the shrinkage and distortion problems which had plagued the production of gold castings up to that point. (Cristobalite is one of the three crystalline configurations of silica. It has unique thermal expansion qualities which makes it especially suitable as an investment material for metal casting.) Even cristobalite investment did not produce perfect castings, and it was not until the 1940's that cristobalite investment materials were formulated that compensated for all of the distortions encountered in the original lost wax technique.